1. Field of the Invention
This invention relates to improved composite materials containing cellulosic pulp fibers dispersed in a polymeric matrix material. The invention also relates to melt-blending and extrusion methods of making these composites and methods of using the same in injection molding applications.
2. Description of the Related Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
In the plastics industry, fillers and reinforcement materials are typically used to improve the properties of plastics. The addition of such materials can improve properties such as conductivity, strength, modulus values, notched impact resistance, etc.
Glass fibers are the most used reinforcement material for thermosets and thermoplastics. Glass fibers impart high strength, dimensional stability, and heat resistance to a plastic composite. Although glass fibers achieve desirable reinforcing properties, glass fibers are costly, abrade processing equipment and increase the density of the plastic systems. In certain applications, these disadvantages outweigh the advantages of using glass fibers as a reinforcement additive.
Cellulosic pulp materials have been evaluated as fillers for plastics in the past. Klason, et al., xe2x80x9cCellulosic Fillers for Thermoplasticsxe2x80x9d, Polymer Composites, (1986); Klason, et al., xe2x80x9cThe Efficiency of Cellulosic Fillers in Common Thermoplastics. Part 1. Filling without processing aids or coupling agentsxe2x80x9d, Intern. J. Polymeric Mater., Volume 10, pgs. 159-187 (1984); Snijder, et al., xe2x80x9cPolyolefins and Engineering Plastics Reinforced with Annual Plant Fibersxe2x80x9d, The Fourth International Conference on Wood Fiber-Plastic Composites, pg. 181-191.
Cellulosic pulp materials have relatively low densities (approximately 1500 kg/m3) and result in reduced wear on the processing equipment compared to glass and mineral materials [e.g., the density of wollastonite, a mineral fiber, is 2900 kg/m3; the density of E(electrical) glass fiber is 2500 kg/m3]. However, prior investigations of the use of wood cellulosic pulps or raw lignocellulosic resources (e.g., wood flour, bagasse) in polymeric materials such as thermoplastics found that a pronounced discoloration of the composite material occurred with the use of these materials at temperatures above 200xc2x0 C. Furthermore, the use of such pulps were found to cause significant off-gasing and disadvantageous odors, principally due to impurities such as lignin. Moreover, previous studies have also found that at temperatures above 200xc2x0 C. the cellulosic fibers themselves had poor reinforcing properties compared even to ground wood and cellulose flours [Klason, et al., Intern. J. Polymeric Mater., Volume 10, p. 175 (1984)]. These disadvantageous results directed previous research efforts to the use of cellulosic materials in polymers having melting temperatures below 200xc2x0 C. such as polypropylene and polyethylene (melting temperatures below 180xc2x0 C.), and away from higher melting temperature materials.
It would be desirable to provide an improved reinforcement filler for use in polymeric materials such as thermoplastics where the filler has a lower cost, lower density, increased reinforcing characteristics, reduced abrasiveness, and the ability to be processed at high temperatures (e.g., above 200xc2x0 C.).
It is an object of the invention to overcome the above-identified deficiencies.
It is another object of the invention to provide an improved composite containing cellulosic pulp materials and methods of making and using the same.
It is a further object of the invention to provide improved composites containing cellulosic pulp materials having reduced discoloration and lower densities.
It is a still further object of the invention to provide extrusion/injection molded products made from the improved composites and methods of making and using the same.
The foregoing and other objects and advantages of the invention will be set forth in or are apparent from the following description.
The inventors of the present application have surprisingly and unexpectedly discovered improved composite materials containing a cellulosic pulp as a reinforcing material. The cellulosic pulp fibers used according to the invention have an alpha-cellulose purity greater than 80% by weight. The use of such cellulosic pulp materials not only provides improved structural characteristics to the composite at a reduced cost and with only a modest increase in the density of the plastic system, but also do not significantly abrade the processing equipment, generate malodors, or result in unacceptable discoloration of the composite. Additionally, the use of the cellulosic pulp materials according to the invention allows for the blending of the components and shaping of the resultant composite material at lower processing temperatures. Surprisingly, the composite materials may be injection molded using processing temperatures below those used with conventional composites, even below the melting point of the polymeric matrix material itself.
One aspect of the invention relates to improved composites containing cellulosic pulp fibers dispersed in a matrix, wherein the matrix comprises a polymeric material and said cellulosic pulp fibers have an alpha-cellulose purity greater than 80% by weight. Preferably, the composite comprises greater than 1% and less than 60% by weight cellulosic pulp fibers, more preferably less than 50% by weight cellulosic pulp fibers, even more preferably, less than 40% by weight and most preferred around 30% or less by weight. Preferably, the fibers are substantially dispersed throughout the composite.
According to one embodiment, the cellulosic pulp fibers have an alpha-cellulose purity greater than 90% by weight, preferably greater than 95% by weight, more preferably greater than 96% by weight, even more preferably greater than 98%.
According to another embodiment, the cellulosic pulp fibers have a lignin content less than 2% by weight, preferably less than 1% by weight, more preferably less than 0.5% by weight.
Suitable purified cellulosic pulps include Ultranier-J, Rayfloc-J-LD, Porosanier-J-HP, Ethenier-F-UHV, Sulfatate-H-J-HD and Placetate-F, each of which are available from Rayonier, Specialty Pulp Products (Jesup, Ga. and Fernandina Beach, Fla.). All of these pulps have an alpha-cellulose purity of 95% or greater with the exception of Rayfloc-J (about 86% alpha-cellulose content). All are softwood pulps with the exception of Sulfatate-H-J which is manufactured from hardwood fibers. The Placetate and Ethenier grades are sulfite pulps whereas the others are kraft pulps. Such pulps are readily available commercially. Other suitable cellulosic pulp materials from other manufacturers include Estercell and Viscocell (International Paperxe2x80x94Natchez, Miss.), Supersoft (International Paperxe2x80x94Texarkana, Tex.), Borregaard UHV-S (Borregaard, Sarpsborg, Norway), Saiccor Acetate and Saiccor Viscose (Saiccor-Umkomass, South Africa), Weyerhaeuser MAC II (Weyerhaeuser, Cosmopolis, Wash.), Buckeye A-5 and Buckeye Cotton Linters (Buckeye Technologiesxe2x80x94Perry, Fla. and Memphis, Tenn., respectively).
The cellulosic pulp fibers may be derived from a softwood pulp source with starting materials such as various pines (Southern pine, White pine, Caribbean pine), Western hemlock, various spruces, (e.g., Sitka Spruce), Douglas fir or mixtures of same and/or from a hardwood pulp source with starting materials such as gum, maple, oak, eucalyptus, poplar, beech, or aspen or mixtures thereof.
Commercial pulps are typically available in sheet form. In order to facilitate the blending of the fibers with the polymeric material, the fiber sheets may be broken down to individual fibers or small aggregates of fibers. According to one embodiment, the cellulosic pulp fibers are granulated so that the fibers can be readily dispersed in the polymeric matrix. The step of granulating may be performed using a rotary knife cutter to break up the pulp material. The granulation process, however, also reduces the length of the fibers. A reduction in fiber length typically decreases the reinforcing impact of a fiber additive. The granulated cellulosic fibers typically have an average length between 0.1 and 6 mm.
According to another embodiment, the cellulosic pulp fibers are pelletized to form pellets of fibers without granulating the fibers since the step of granulating the cellulose fibers prior to blending with the polymeric material decreases fiber length. Replacing the granulating step with the use of pelletized fibers preserves the fiber length to a much greater extent, and also allows for adequate feeding and mixing of the fibers with the polymer. With greater fiber length retention, the tensile strength and unnotched Izod impact properties of the composites are substantially enhanced. The fibers in pelletized form are mixed with the polymeric matrix material and fed to the extruder. During melt blending, the pellets break down allowing the individual fibers to be readily dispersed throughout the matrix. Advantageously, the pelletizing of the fibers does not significantly decrease the fiber length which increases the reinforcing properties of the fibers.
The fiber length may also be reduced during the blending operation. Accordingly, one preferred embodiment involves introducing the pellets at a later stage of the blending process. For example, the fibers may be introduced at one of the latter zones of a twin screw extruder to enable sufficient blending with the polymer without a significant reduction in fiber length.
The matrix material of the composite comprises a polymeric material melting preferably between 180-270xc2x0 C. Suitable polymeric materials include polyamides, PET (polyethylene terephthalate), PBT (polybutylene terephthalate), or mixtures thereof. Other suitable materials include PTT (polytrimethylterephthalate) (eg. corterra by Shell), ECM (ethylene-carbon monoxide) (eg. Carilon by Shell) and styrene copolymer blends such as styrene/acrylonitrile (SAN) and styrene/maleic anhydride (SMA) thermoplastic polymers. Still further materials include polyacetals, cellulose butyrate, ABS (acrylonitrilexe2x80x94butadiene-styrene), certain methyl methacrylates, and polychlorotrifluoroethylene polymers.
According to one preferred embodiment, the polymeric material is a thermoplastic having a melting point greater than 180xc2x0 C., more preferably greater than 200xc2x0 C., and even more preferred between 220-250xc2x0 C. Preferably, the polymeric material is a thermoplastic selected from nylon 6, nylon 12, nylon 66 or mixtures thereof.
According to another embodiment, the composite may comprise a thermally sensitive additive which is sensitive to elevated processing temperatures to result in a novel composite product. Since the polymer/fiber composites of the invention can be processed at lower temperatures, thermally sensitive additives otherwise incompatible with a polymeric material due to high processing temperatures can be employed. For example, an additive which can not be employed in a nylon injection molding application because of the high temperatures conventionally employed may be used with nylon according to the present invention since lower processing temperatures may be used. The result is a novel injection molded product having improved properties and characteristics. Suitable thermally sensitive additives include anti-microbial compounds, colorants, and fragrances.
The reinforced composites according to the invention have improved properties and characteristics. Advantageously, the composite of the invention has improved properties with only modest density increases of the polymeric matrix material. More specifically, the cellulosic fibers of the invention can be added to the polymeric matrix material without major increases in the density of the resulting composite material, unlike what occurs when reinforced with equivalent amounts by weight of the more dense glass fiber or mineral material alternates (e.g., see Table IVA in Example 1)
According to one embodiment, the composite has a density less than 30% different from the unfilled polymeric material and a tensile strength 10% greater than that of the unfilled polymeric material. More preferably, the composite has a density less than 20% different from and a tensile strength 20% greater than that of the polymeric material. According to another embodiment, the composite has a density less than 30% different from and a tensile modulus 50% greater than that of the polymeric material. Preferably, the composite has a density less than 20% different from and a tensile modulus 80% greater than that of the polymeric material.
According to yet another embodiment, the composite has a density less than 30% different from and a flexural strength 25% greater than that of the polymeric material. Preferably, the composite has a density less than 20% different from and a flexural strength 45% greater than that of the polymeric material.
According to a still further embodiment, the composite has a density less than 30% different from and a notched Izod impact strength less then 30% different from that of the polymeric material. Preferably, the composite has a density less than 20% different from and a notched Izod impact strength equal to or greater than that of the polymeric material.
Another advantage of the invention is the reduced discoloration in the resultant composite. Prior use of pulp fibers typically resulted in substantial or severe discoloration of the final product. This discoloration is significantly reduced or avoided using the present invention.
The composite may further comprise at least one coupling agent or compatibilizer. Suitable agents include titanates, zirconates or mixtures thereof. Preferably, the coupling agent is present in an amount greater than 0.0001% and less than 3% by weight, more preferably in an amount less than 2% by weight.
The composite may further comprise at least one colorant to alter the color of the composite. Suitable colorants include carbon black, TiO2 and the like.
Another embodiment of the invention relates to a composite comprising at least 5 wt % fibers dispersed in a matrix comprising a polymeric material, wherein the composite has a density less than 5% greater than the polymeric material and a tensile strength 2% greater than the tensile strength of said polymeric material. Preferably, the composite has a density less than 2% greater than the polymeric material and a tensile strength 3% greater than the tensile strength of said polymeric material.
Another aspect of the invention relates to methods of making the improved composite material comprising the cellulosic pulp fibers and the polymeric material. According to one embodiment, the mixture is formed by blending granules of the polymeric material with the pulp fibers to form a composite blend. The pulp fibers may be granulated fibers or pelletized fibers. The polymeric material may be in the form of granules, pellets, particulates, fibers or the like.
One embodiment relates to a method of making a composite material comprising the steps of:
(a) forming a mixture comprising cellulosic pulp fibers and polymeric material; and
(b) melt blending said mixture to form said composite material;
wherein said cellulosic pulp fibers has an alpha-cellulose purity greater than 80% by weight.
The polymeric/pulp fiber mixture should have a moisture content less than 5% by weight, preferably less than 1% by weight and/or is substantially free of solvent. Preferably, the cellulosic pulp fibers are dried prior to said blending.
According to one embodiment, the method further comprises the step of granulating the pulp material prior to forming the mixture. Suitable granulating devices include a rotary knife cutter.
According to another embodiment, the method further comprises the step of pelletizing the fibers to form pellets of the fibers prior to forming the mixture.
One preferred embodiment relates to a method comprising melt blending/extruding the mixture of polymeric material and cellulosic pulp fibers to form an extruded composite. Preferably, the melt blending/extruding is achieved using a twin-screw extruder.
One surprising advantage resulting from the invention is the ability to melt blend the polymeric material with the pulp fibers at lower temperatures. Preferably, the blending is at a blending temperature below the melting temperature of the polymeric material, more preferably, the blending is at a blending temperature at least 10xc2x0 F. less than the melting temperature of the polymeric material, even more preferably at least 20xc2x0 F. less, even more preferably at least 30xc2x0 F. less and most preferred at least 50xc2x0 F. less.
The method of the invention may further comprise the step of comminuting the composite blend to form composite granules suitable for use in applications such as injection molding, melt extrusion, melt pultrusion, etc.
Another aspect of the invention relates to the use of the composite materials of the invention to form an injection molded product. Accordingly, one embodiment of the invention relates to a method comprising the step of injection molding the composite material to form an injected molded product. Preferably, the method comprises injection molding granules of the composite.
Surprisingly, the use of the cellulosic pulp fibers of the invention enables injection molding to occur at reduced temperatures relative to injection molding composites containing fillers such as glass fibers and mineral fillers (e.g., wollastonite).
According to one embodiment, the injection molding is at processing temperatures below the processing temperature for molding glass- and mineral-filled polymers. Preferably the injection molding processing temperature is at least 20xc2x0 F. less than the melting temperature of the polymeric material, more preferably at least 30xc2x0 F. less than the melting temperature of the polymeric material, more preferably at least 40xc2x0 F. less than the melting temperature of the polymeric material and most preferably at least 50xc2x0 F. less than the melting point of the polymeric material. In the examples shown in Table VI, the cellulose fiber composites were injection molded at nozzle and barrel zone temperatures approximately 100xc2x0 F. less than those for the glass- and mineral-filled polymers.
Yet another aspect of the invention relates to methods of using the improved composites to form improved products such as composite granules and injection molded products.
One embodiment of the invention relates to a composite granule composed of fiber-reinforced polymeric material comprising a multiplicity of the cellulosic pulp fibers dispersed in a matrix of thermoplastic material. Preferably, the granules have a largest dimension less than 10 mm.
Another embodiment of the invention relates to an injection molded product of the fiber-reinforced thermoplastic material comprising the cellulosic pulp fibers dispersed in a matrix of the thermoplastic material. The injection molded product may have a complex shape with multiple sharp corner radii.